Currently, a display device which is applicable to mobile electrical apparatus, such as a vehicle navigation device or a mobile phone, has a brightness regulation function for regulating a display brightness thereof according to the brightness of the ambient light. For example, Japan Patent Publication No. 2001-522058 discloses a display system including a controller to change the brightness of a display according to the ambient light detected by a photo-sensor. By using the brightness regulation function, the brightness of the display is raised under a strong ambient light environment, such as in daytime or being outdoors, and the brightness of the display is reduced under a weak ambient light environment, such as at night or being indoors.
In general, for detecting the ambient light, a display device includes a photo-sensor to detect light and output a photocurrent corresponding to the light-acceptance amount. The photocurrent can be transformed into a digital pulse signal by using a signal converter, such as a current-to-voltage converter or an analog-to-digital converter. A controller regulates the brightness of a backlight source corresponding to the inputted signal. A circuit used for light detecting may be disclosed in JP Patent Pub. No. 2008-522159.
However, considering an indoor environment using a fluorescent lamp with a driving frequency of 50 or 60 Hz, since the light intensity, which is detected by the photo-sensor of the conventional display device including the above-mentioned circuit, varies with the driving frequency of the fluorescent lamp, it is a problem that the output of the photo-sensor also varies with the driving frequency.